In conventional digital-to-analog converters, a first conversion stage is followed by multiple stages of low-pass filtering to filter out unwanted noise. In one type of digital-to-analog converter, a digital delta-sigma modulator is utilized. The delta-sigma modulator receives a digital input and converts it into a one-bit digital output. This output is typically passed through a one-bit DAC and then into an active RC low pass filter. The active RC low pass filter utilizes a series of resistors and various active components in order to realize the desired filter function. The disadvantage to this type of filter is the sensitivity of the filter to variations in the components. A significant amount of trimming is often required.
In another type of filtering scheme, a switched-capacitor filter has been optimized. However, noise performance of this type of filter has not been optimized to its fullest due to the fact that switched-capacitor filters in DACs utilizing delta-sigma modulators have been realized with cascaded bi-quad switched-capacitor filters. The disadvantage to this is that the multiple bi-quad stages add a significant noise component to the overall filter function, which is undesirable.
A problem exists with conventional digital-to-analog converters utilizing any type of switched capacitor filter is the requirement for conversion from the sampled data domain on the output of the switched capacitor filter to a continuous time domain. In the past, an active filter has been utilized to directly convert between the sampled data output by the switched capacitor and a continuous time output. However, this conversion typically results in a high degree of distortion that is added to the signal. Any distortion, especially in low level DACs, is detrimental to the overall performance.